This invention is concerned with magnetization of ferromagnetic material in elongated objects such as fiber optic cables and is more particularly concerned with providing cables and pipes with permanent magnetic signatures to facilitate their detection magnetically.
Many types of buried pipelines and power and communication cables are either fabricated of ferromagnetic material or employ ferromagnetic material as strength or armor members, making them susceptible to detection magnetically. Larger pipes and cables may, to a certain extent, be detectable magnetically by virtue of their natural magnetization (e.g., due to the earth""s magnetic field), but smaller pipes and cables are more difficult to detect magnetically.
Submarine fiber optic communication cables, which are strung by ships across the oceans of the world, must be buried beneath the seabed for protection from sea life, anchors, and trawls, but subsequently they must be located for repairs or maintenance. Conventional means of locating, tracking and determining burial depth of unpowered buried fiber optic communications cables magnetically are limited in capability. Smaller cables in current use are difficult to detect by passive magnetization techniques at slant ranges greater than roughly 0.8 meters. Nevertheless, projected world-wide expansion of the subsea fiber optic communications networks will result in a greater proportion of small cables which are the most difficult to detect.
The present invention provides fiber optic cables, and other elongated objects comprising ferromagnetic material, with enhanced permanent magnetic signatures which allow detection, location, tracking and burial depth determination of the objects at unprecedented slant ranges. The invention involves unique methods and apparatus for producing magnetization in ferromagnetic material of elongated objects and provides the objects with strong distinctive magnetic signatures. The invention will be described in its application to fiber optic communications cables, such as those employed in subsea fiber optic communications networks, but it will be apparent that the invention is not limited to such applications and may be employed, more generally, with regard to cables, pipes, or other elongated objects comprising ferromagnetic material.
Magnetization of unpowered fiber optic cables in accordance with the invention can be achieved during the cable laying process, or during the manufacture of the cables, for example, and does not require changes in cable construction or the laying process itself, nor does it interfere in any way with the operation of the fiber optic cable or cause mechanical stress or damage to the cable.
Submarine fiber optic communications cables typically have an optic fiber surrounded by a steel wire strength rope or steel armor. It is known that such cables may have unintended natural magnetization resulting from local magnetic fields (primarily the earth""s field) captured at the time of fabrication, but such naturally occurring magnetization of both armored and unarmored cables is generally far less than the saturation field level of the ferromagnetic materials in the cable and is not easily detected. Moreover, the external magnetic fields resulting from naturally-occurring magnetization tend to follow the helical arrangement of the armor or wire ropes and are not uniformly radial or cylindrically symmetric. Passive magnetic detection of the external fields is quite limited. For example, with available detection equipment, burial depth measurement is limited to about 0.9 to 1.2 meters on 40 mm cable, and to about 0.5 to 0.8 meter on 10 mm cable. Continuous tracking and burial depth measurements are made difficult by variations in magnetic field strength along the cables associated with the pitch length of armor or wire ropes.
The present invention produces magnetic fields near saturation in ferromagnetic material of fiber optic cables, or other elongated objects, inducing permanent remnant magnetization. The produced magnetization is much stronger than any natural magnetization of the ferromagnetic material by the earth""s magnetic field. An axial gradient in the applied axial (longitudinal) magnetization produces a radial external xe2x80x9cleakagexe2x80x9d magnetic field around the cable that is substantially cylindrically symmetric and that varies periodically along the length of the cable, providing a strong permanent magnetic signature that can be readily detected by passive magnetic detection methods. The periodic variations in the radial external magnetic field along the cable may have a square wave or a sine wave pattern, for example. If the wavelength of the variations is long enough, the external field strength over most of the cable decreases approximately linearly with distance from the cable.
The desired magnetization can be achieved by the use of a unique magnetizer having a pair of mirror-image magnets (or a plurality of pairs) disposed at opposite sides of the cable, for producing a strong magnetic field adjacent to the cable, which varies repetitively as the cable is moved longitudinally relative to the magnets.